JACK 5.4 Application Interface

aos.jack.jak.cursor
Class FalseCursor

java.lang.Object
  extended byaos.jack.jak.util.Watchable
      extended byaos.jack.jak.cursor.Cursor
          extended byaos.jack.jak.cursor.FalseCursor
All Implemented Interfaces:
ObservableInt

public class FalseCursor
extends Cursor

Utility cursor that always returns false. Useful for code that needs to be able to return a cursor indicating failure.


Field Summary
 
Fields inherited from class aos.jack.jak.cursor.Cursor
FALSE, falseCursor, TRUE, trueCursor, UNKNOWN
 
Constructor Summary
FalseCursor()
           
 
Method Summary
 void finished()
          This informs the cursor that it is not going to be used again.
 boolean next()
          This method is called to provide a next binding by this cursor.
 Cursor reset(aos.jack.jak.core.Generator g, int index)
          Resets the cursor.
 void undo()
          Undo tells the cursor to backtrack anything it has done.
 boolean unRegister(java.util.Observer o)
           
 
Methods inherited from class aos.jack.jak.cursor.Cursor
isTriggered, negate, register
 
Methods inherited from class aos.jack.jak.util.Watchable
addObserver, countObservers, deleteObserver, deleteObservers, hasChanged, notifyObservers, notifyObservers
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Constructor Detail

FalseCursor

public FalseCursor()
Method Detail

next

public boolean next()
Description copied from class: Cursor
This method is called to provide a next binding by this cursor. The first call provides the first binding, and successive calls provide successive bindings, until the binding alternatives are exhausted. The method returns true when it provides a binding, and false when it cannot provide a new alternative binding. Expected behaviour is that cursors will return while there are more bindings and from then on false until a call to reset(). This is very important especially when you consider an expression such as:
     db.query($x) && isWhatIWant($x) && db1.query($x, $y)
 
If the isWhatIWant() cursor simply tests its input and returns true for appropriate values then this expression would not work as expected and may go into an infinite loop.
For example if the beliefset had:
                db(1)
                db(2)
                db(3)

                db1(3,3)
 
and isWhatIWant() returned true on odd numbers. Then the correct behaviour would be one result with $x=3 $y=3. However if isWhatIWant.next() always returns true on odd numbers then: $x is bound to 1,
isWhatIWant(1).next() is true
db1(1, $y) fails, backtracks
isWhatIWant(1).next() is true
db1(1, $y) fails, backtracks
etc...

Specified by:
next in class Cursor

reset

public Cursor reset(aos.jack.jak.core.Generator g,
                    int index)
Description copied from class: Cursor
Resets the cursor. This is normally done by either undoing the last action or finishing the current cursor and returning a new one.

Overrides:
reset in class Cursor
Parameters:
g - The generator that generates an equivalent cursor
index - The index of the equivalent cursor in the generator.
Returns:
The reset cursor, or an equivalent one.

undo

public void undo()
Description copied from class: Cursor
Undo tells the cursor to backtrack anything it has done. It is a subset of the reset() functionality since it does not reinitialize the cursor. The only requirement is that any bindings made by the cursor are undone. The state of the cursor after the call is undefined. Most implementations will fail if next() is called after undo() without an intervening call to reset().

Overrides:
undo in class Cursor

finished

public void finished()
Description copied from class: Cursor
This informs the cursor that it is not going to be used again. This is typically used to clean up data strutures and to remove itself from any notification paths.

Overrides:
finished in class Cursor

unRegister

public boolean unRegister(java.util.Observer o)
Overrides:
unRegister in class Cursor

JACK 5.4 Application Interface

Copyright (C) 1999-2008, Agent Oriented Software Pty. Ltd.

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The JACK Modules and relevant Software Material have been developed entirely at private expense and are accordingly provided with RESTRICTED RIGHTS. Use, duplication, or disclosure by Government is subject to restrictions as set forth in subparagraph (c)(1)(ii) of DFARS 252.227-7013 or subparagraph (c)(1) and (2) of the Commercial Computer Software Restricted Rights and 48 CFR 52.2270-19, as applicable.


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